In optical instruments it is essential that the lenses of an instrument have a well defined position in their lens holders and that the instrument has low weight. For instruments subjected to temperature variations, problems often occur due to the coefficient of linear expansion of the lens differing from that of the lens holder. There can be play between lens and holder, or the lens can be subjected to compressive stresses and be deformed so that its optical performance is changed. For example, there is a diameter difference of 0.17 mm between the lens and holder for a glass lens of 110 mm diameter and a holder of aluminum for a difference in temperature of 110 degrees. The permitted radial movement of the lens will only be 0.024 mm, however, for a focal length of 120 mm and a permitted angular deviation of 0.2 mrad in the lens. It has been proposed that the lens holder is made from a material having a coefficient of linear expansion which closely agrees with that of the lens. Titanium is a suitable material for a glass lens, but this material is expensive and considerably heavier than aluminum, for example. It has also been proposed that the lens is fastened in a radially resilient holder, as illustrated in the Japanese Patent No. 59-31914. The accuracy of this type of lens holder has been found insufficient in many instrument applications, however. An alternative resilient lens holder for a plastics lens is shown in the U.S. Pat. No. 4,506,951. In this holder the lens is formed with a thin edge part extending round the lens, and which is elastically deformable in a radial direction. This lens holder cannot be applied to a glass lens, however.